1. Field of the Invention
The invention is directed to electrical switching apparatus and, more particularly, to electrical switching apparatus such as, for example, receptacles including terminals, such as screw terminals, for electrical conductors, such as copper wiring.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices and circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers and other load controllers.
Circuit breakers are generally old and well known in the art. An example of a circuit breaker is disclosed in U.S. Pat. No. 5,341,191. Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. Molded case circuit breakers, for example, include at least one pair of separable contacts which are operated either manually by way of a handle disposed on the outside of the case or automatically by way of an internal trip unit in response to an overcurrent condition.
Ground fault circuit interrupters (GFCIs) include ground fault circuit breakers (GFCBs), ground fault switches and other ground fault contactors, motor starters, motor controllers and other load controllers.
Arc fault circuit interrupters (AFCIs) include arc fault circuit breakers (AFCBs), arc fault switches and other arc fault contactors, motor starters, motor controllers and other load controllers.
Ground fault and/or arc fault switches include ground fault and/or arc fault receptacles (GFRs/AFRs), and cord-mounted or plug-mounted ground fault and/or arc fault protection devices (e.g., ground fault and/or arc fault protection circuitry at the alternating current (AC) plug end of the AC power cord of an appliance, such as a hair dryer).
A typical GFCI includes an operational amplifier, which amplifies a sensed ground fault signal and applies the amplified signal to a window comparator. The window comparator compares the amplified signal to positive and negative reference values. If either reference value is exceeded in magnitude, a trip signal is generated.
A GFCI may employ, for example, the well known dormant oscillator technique for sensing a load side grounded-neutral condition, without requiring a connected load. Two magnetic elements are employed. The first magnetic element is a differential current transformer, which produces an output proportional to the difference in the current flowing to the load through the line conductor and the current returning from the load through the neutral conductor. The difference is the ground current. The second magnetic element is a voltage transformer, the primary of which is energized by the output of a ground fault sense amplifier, which is part of the GFCI electronics. The transformer has two single turn secondaries formed by passing line and neutral conductors through its core. The polarities of the primary and secondary windings of the transformer are such that the ground fault sense amplifier output induces a voltage on the secondary of transformer, such as the neutral conductor, which voltage increases the ground current caused by a load neutral-to-ground fault. This increased ground current increases the sense amplifier output, thereby resulting in a positive feedback condition increase in the ground current. If the load neutral-to-ground impedance is less than about 2 ohms, this positive feedback may become unstable, which results in a monotonic increase in the induced ground fault current in the neutral conductor until the ground fault trip level is exceeded and the receptacle trips. Both conductors are passed through the core to cover the case where the input leads are reversed.
A glowing contact is a high resistance connection, which can form at the interface of a copper wire and a screw terminal, for example, of a receptacle. The resulting temperature rise at this connection point can melt the wire""s insulation and damage the receptacle. It is desirable to be able to detect this condition and interrupt the current before the glowing contact fault progresses to a hazardous condition.
The hazard associated with aluminum wiring has been known and understood for thirty years. The connection of an aluminum wire conductor to the terminal of a wiring device is unstable, since the aluminum, over time, tends to flow, thus, making the aluminum wire-to-terminal a high resistance connection. The resulting I2R heating causes local heating that can melt the wire""s insulation and the receptacle. It was believed that simply returning to copper wire would resolve this problem. Unfortunately, this is not true. Furthermore, most people, outside of the standards and wiring device industry, are unaware of the glowing contact problem. Also, the lack of wide spread public knowledge of the glowing contact problem may follow from the fact that there has been no known solution to this problem.
It is very easy to create a high resistance or glowing contact at a receptacle terminal using copper wire. See, for example, Sletbak, J., et al., xe2x80x9cGlowing Contact Areas in Loose Copper Wire Connections,xe2x80x9d IEEE, 1991, pp. 244-48.
The hazards associated with glowing contacts, including contacts made with all combinations of copper, brass and iron are known. See Yasuaki Hagimoto, xe2x80x9cJapanese Reports on Electrical Fire Causes,xe2x80x9d http://members.ozemail.com.au/xcx9ctcforen/japan/index.html, 1996, 12 pp.
In a similar manner that aluminum oxide creates the aluminum wire problem, the culprit associated with a glowing contact is copper oxide. There are two recognized mechanisms for creating a high resistance copper oxide contact: arcing; and fretting. The arcing mechanism involves, for example, a loose receptacle screw terminal and slight movement of the wire while it is carrying a current. Every time the electrical connection is broken, a single electrical arc discharge can occur.
FIG. 1 shows the voltage across the terminal-to-wire connection in the upper trace (about 170 V peak) and the current through that connection in the lower trace (about 15 A peak) for different intervals of an electrical connection being broken while carrying current. This pair of voltage and current traces is broken into three intervals I,II,III. The first interval I shows normal operation in which there is negligible voltage across the terminal-to-wire connection, which has a relatively low resistance, with an alternating current flowing through that connection. During the second interval II, there is a significant increase in the resistance of the terminal-to-wire connection, due to a single arcing half cycle. Hence, there is a corresponding significant increase in the voltage across the terminal-to-wire connection, along with a corresponding reduction in the magnitude of the alternating current flowing through that connection. Finally, during the third interval III, the terminal-to-wire connection becomes an open circuit and the voltage across the terminal-to-wire connection is the line voltage. As a result of the open circuit, there is essentially no current flowing through that connection.
While there is essentially very little power dissipated in the terminal-to-wire connection during the first and third intervals I,III, relatively significant arcing and power dissipation occurs in the second interval II. To the extent that the second interval II may become relatively periodic or persistent, then oxidation can occur at the copper wire-screw interface where the half cycle arcing has occurred with each breaking of the wire-screw connection. This copper oxide layer at the wire-screw interface can also occur due to the mechanism of fretting or a rubbing action with no arcing.
By Paschen""s laws, it is not possible to create a sustained copper-to-copper through air arc discharge in a 120 VRMS circuit with a resistive load. An arc is formed when the contact breaks, although it extinguishes at the first zero current crossing, since the voltage is too small for a xe2x80x9cre-strikexe2x80x9d. This is sometimes called a xe2x80x9csparkxe2x80x9d rather than an xe2x80x9carcxe2x80x9d. There can be a spark whenever an electrical contact is broken due to local heating at the break point. Hence, an inductive load is needed for an arc in most 120 VRMS residential wiring, other than a 240 VRMS circuit. Otherwise, with a resistive load, a peak voltage of about 300 volts is needed in order to create a sustained arcing event as compared to an available peak value of about 170 volts for a 120 VRMS circuit.
Each single arc discharge forms a small amount of copper oxide (Cu2O) at the terminal-to-copper wire interface. With repeated discharges, the amount of the copper oxide increases over time. Copper oxide has a number of characteristics which, when combined, creates a hazard. First, the interface can be mechanically strong. Hence, once the terminal-to-copper wire connection is made through the copper oxide, the connection may become permanent. Second, copper oxide is a semiconductor that has a very high negative resistance-versus-temperature characteristic between about 180xc2x0 C. and about 250xc2x0 C. Over this temperature range, the resistance decreases as much as five orders of magnitude. As the connection heats, the current tends to concentrate into a relatively narrow region, thereby resulting in a very high current density and temperature. For example, a temperature of about 1200xc2x0 C. to about 1300xc2x0 C. may result, which temperature is hot enough to melt, for example, a receptacle""s plastic housing, but not the copper oxide. Then, as the terminal heats, the wire insulation begins to fail.
During a glowing contact fault in a receptacle, the copper wire reaches a glowing temperature value at which time the wire looks like an electric heater coil. First, the wire""s insulation melts at the terminal and, then, slowly progresses away from the terminal toward other wires in the receptacle""s outlet box. This can result in either an arcing fault or a ground fault if the bare glowing wire contacts another conductor. Second, the heat resulting from the glowing contact fault flows into the receptacle and causes the plastic housing of the receptacle to melt. As the plastic melts, the receptacle loses its mechanical integrity and, thus, the electrical isolation between conductors is compromised. This may ultimately lead to either a line-to-ground fault or a neutral-to-ground fault. In the event that the upstream protective device (e.g., a circuit breaker) does not respond, then the plastic could ignite.
FIGS. 2 and 3 show respective representations of visible and infrared photographs of glowing contacts GCV and GCIR.
Once a glowing contact is formed, the current during the formation of the glowing contact and the subsequent current flowing through the glowing contact is typically normal, since the voltage drop across a glowing contact is typically about 2 VAC. The existence of a glowing contact, therefore, is not reliably detectable by a conventional upstream current protective device (e.g., a conventional circuit breaker or fuse). However, significant damage may result to both the wire""s insulation and the receptacle. On the other hand, if an upstream circuit breaker with both arc and ground fault protection is employed, then that circuit breaker will respond to arcing or a ground fault resulting from insulation damage caused by a glowing contact and will eventually trip in order to de-energize the branch circuit, thereby protecting the damaged wire and/or receptacle.
There exists the need to provide protection from a glowing contact in an electrical switching device, such as a receptacle, in order to provide protection should an upsteam branch circuit protective device not include arc and ground fault protection.
These needs and others are met by the present invention, in which dual temperature sensors output signals representative of the temperature of line and neutral circuits. A protection circuit then determines a difference between those two signals and provides a trip signal as a function of the difference.
As one aspect of the invention, an electrical switching device comprises: a line circuit having a first temperature; a neutral circuit having a second temperature; a load terminal; separable contacts adapted to electrically connect the line circuit and the load terminal; an operating mechanism for opening the separable contacts in response to a trip signal; a first temperature sensor outputting a first signal representative of the first temperature of the line circuit; a second temperature sensor outputting a second signal representative of the second temperature of the neutral circuit; means for determining a difference between the first and second signals; and means for providing the trip signal as a function of the difference.
The means for providing may comprise a comparator, which outputs the trip signal when the difference exceeds a predetermined value.
The line circuit may include a line terminal, and the first temperature sensor may be a diode disposed proximate the line terminal. The neutral circuit may include a neutral terminal, and the second temperature sensor may be a diode disposed proximate the neutral terminal.
The means for determining a difference may comprise a first resistor, a second resistor, a third resistor, and a differential amplifier having a first input, a second input and an output. The first resistor is electrically connected between the first temperature sensor and the first input of the differential amplifier. The second resistor is electrically connected between the second temperature sensor and the second input of the differential amplifier. The third resistor is electrically connected between the output of the differential amplifier and one of the first and second inputs of the differential amplifier.
The means for providing may comprise a window comparator having a first reference, a second reference, an input inputting the difference, and an output having the trip signal, which is active when the difference is greater than the first reference or less than the second reference.
The window comparator may comprise a first diode; a second diode; a first comparator having a first input, a second input, and an output; and a second comparator having a first input, a second input, and an output. The first input of the first comparator inputs the first reference, the second input of the second comparator inputs the second reference, the second input of the first comparator and the first input of the second comparator input the difference, the first diode is electrically connected between the output of the first comparator and the output of the window comparator, the second diode is electrically connected between the output of the second comparator and the output of the window comparator.
The means for providing may comprise a window comparator having an input electrically connected with an output of the differential amplifier, a first reference voltage and a second reference voltage. The first and second reference voltages define a voltage window with the first reference voltage being above a nominal voltage of the differential amplifier and the second reference voltage being below the nominal voltage of the differential amplifier.
As another aspect of the invention, a trip circuit is for an electrical switching device including a line circuit having a first temperature, a neutral circuit having a second temperature, and a load terminal. The trip circuit comprises: a first temperature sensor outputting a first signal representative of the first temperature of the line circuit; a second temperature sensor outputting a second signal representative of the second temperature of the neutral circuit; means for determining a difference between the first and second signals; and means for providing the trip signal as a function of the difference.
As a further aspect of the invention, an electrical switching device comprises: a line terminal having a first temperature; separable contacts; a first conductor electrically connecting the line terminal and the separable contacts; a load terminal having a second temperature; a second conductor electrically connecting the separable contacts and the load terminal; an operating mechanism for opening the separable contacts in response to a trip signal; and means for providing the trip signal as a function of a difference between the first temperature and the second temperature.